The present invention relates to a spray powder, a thermal spraying process using it and a sprayed coating. More particularly, the present invention relates to a spray powder which is capable of showing high deposition efficiency and which is capable of forming a sprayed coating having extremely high toughness and impact resistance as compared with conventional products and also having excellent corrosion resistance and wear resistance in a wet environment, and a thermal spraying process employing it and a sprayed coating.
Metal parts of various industrial machines or general-purpose machines are required to have various properties such as corrosion resistance, wear resistance and heat resistance depending upon the respective purposes. However, in many cases, the metals can not adequately satisfy the required properties by themselves, and it is often attempted to solve such problems by surface modification. A thermal spraying process is one of surface modification techniques which are practically used, as well as physical vapor deposition or chemical vapor deposition. Thermal spraying has characteristics such that the size of a substrate is not limited, a uniform sprayed coating can be formed on a substrate having a large surface area, the speed of forming the coating is high, its application on site is easy, and a thick coating can be formed relatively easily. In recent years, its application has been expanded to various industries, and it has become an extremely important surface modification technique.
With respect to a thermal spraying process, various techniques have been developed. Among them, high velocity flame spraying has characteristics such that the particle velocity is high, and the particles will impinge on a substrate at a high speed, whereby a highly dense coating having a high adhesion to the substrate can be obtained, inclusion of atmospheric air into the flame is relatively small, and yet the particle velocity is large, whereby the dwelling time in the flame is short, overheating of particles is less, and modification of the spray material is little.
As a spray material, WC has extremely high hardness and is excellent in wear resistance. However, spraying of WC alone is difficult. Usually, WC is used as mixed or complexed with a metal such as Co or Ni, or an alloy containing such a metal, as a binder. A sprayed coating formed from a WC/chromium carbide/Ni or Ni-based alloy spray powder using a Ni or Ni-based alloy as a binder, shows excellent corrosion resistance and wear resistance in a wet environment and thus is being widely employed.
However, a sprayed coating formed by using the above spray powder has a problem that it is poor in toughness and impact resistance. Specifically, such a spray powder is often sprayed to parts which will be used in a wet environment, and if the sprayed coating receives a substantial impact during its use, the coating will have cracks, which tend to cause peeling of the coating from the substrate. If this happens, the useful life of the product will be short, and the application of the sprayed coating will be limited.
The present inventors have conducted extensive researches to solve the above problems and as a result, have found that it is possible to obtain a spray powder which shows high deposition efficiency and which is capable of forming a sprayed coating having extremely high toughness and impact resistance and also having excellent corrosion resistance and wear resistance in a wet environment, by agglomeration and sintering of material powders (WC, a chromium carbide and a Ni or Ni-based alloy) having their particle sizes adjusted within proper ranges. The present invention has been accomplished on the basis of this discovery.
That is, in order to solve the above problems, the present invention provides a spray powder which has a particle size of from 6 to 63 xcexcm and which comprises from 75 to 95 wt % of a ceramic phase made of a WC powder and at least one chromium carbide powder selected from the group consisting of Cr3C2, Cr7C3 and Cr23C6, and from 5 to 25 wt % of a metal phase made of a Ni or Ni-based alloy powder, wherein the mean particle size of primary particles of the WC powder constituting the ceramic phase is from 5 to 20 xcexcm. and the mean particle size of primary particles of the chromium carbide powder is from 1 to 10 xcexcm.
Further, the present invention provides a thermal spraying process which comprises carrying out high velocity flame spraying using such a spray powder, and a sprayed coating which is formed by carrying out high velocity flame spraying using such a spray powder and which comprises from 75 to 95 wt % of a ceramic phase made of the WC powder and the chromium carbide powder, and from 5 to 25 wt % of a metal phase made of the Ni or Ni-based alloy powder, wherein the mean particle size of primary particles of the WC powder constituting the ceramic phase is from 5 to 20 xcexcm. and the mean particle size of primary particles of the chromium carbide powder is from 1 to 10 xcexcm.